Actuated miniaturized process valves for high pressures, for example, pressures higher than 300 bar, are usually constructed of a valve body, an electric or pneumatic actuator and a connecting mounting unit. This makes it necessary for the valves to have spatially large forms of construction. If the valve is required to assume a defined safety (i.e., “fail safe”) position in the event of the failure of control energy, the valve actuators and consequently the controllable valve become disproportionably larger. For automated and miniaturized test installations, actuated gas-tight valves with spatial dimensions smaller than 50 ml are required, so that a compact installation set-up is possible. For this reason, in particular, known actuated control valves are too large, and therefore unsuitable, for use in miniaturized and automated test installations. All known valves are very similar in terms of construction.
The construction of known valves varies, as a rule, in terms of the configuration of the sealing seat, the form of the valve tappet and valve sealing seat and also the material combination of the valve tappet and valve sealing seat. Control valves which are normally primarily intended for a continuous regulating function do not always require high degrees of leak-tightness in the closed position. In many production plants, therefore, auxiliary downstream and upstream leak-tight OPEN/SHUT fittings are installed.
Operational valves, in the closed position, must be leak-tight in the product passage and relative to the outside. The valve seat and the valve-spindle seal therefore have to satisfy leak-tightness requirements, particularly where a large number of switching cycles are concerned. The two necessary sealing points important for functioning require a high sealing force in order to seal against high process pressures. The sum of the sealing force and the frictional force in the valve determine the size of the pneumatic drive which is necessary. This leads, as a rule, to very large actuating drives.
One possibility for the design of a pressure-resistant valve is described in the published specification EP 742 398 A1.
This valve has a cavity which is filled with compressible supporting medium and through which the valve spindle is moved and which is separated from the flow passage of the valve by means of an elastic diaphragm.
The disadvantage of such a design is that, over the course of a comparatively large number of actuations of the valve (>10,000 switching cycles), the diaphragm may become leaky or may tear. The product then leaks from the flow passage or valve process space into the interspace between the valve spindle and valve body and ultimately even possibly into the surroundings.
Valves have to execute long travels for a complete closing movement between the end positions OPEN and SHUT, and therefore the construction of a spindle seal, particularly where prevailing high process pressures are concerned, becomes complicated and expensive. For example, multi-part or multiple seals with the possibility of resilient adjustment are employed. The possibility of adjustment of a spindle seal compensates for the seal abrasion occurring during the to-and-fro movement of the spindle, so that the spindle seal is pressed down by a resilient component and continues to remain leak-tight. If a valve with a seal design of this type is used, the service life of the seal under continuous process conditions and with a low frequency of movement of the valve spindle is very long, because there is virtually no wear at the spindle seal. If, however, this valve is used for a task in which the full actuating range (actuating travel) is covered constantly, so that the valve is constantly moved between the end positions OPEN and SHUT, pronounced abrasion occurs at the spindle seal, so that a design-based adjustment possibility is quickly used up, the spindle seal is no longer leak-tight and a leakage occurs. The result of this is that such designs of spindle seals are not suitable in valves for high process pressures, such as, for example, 300 bar hydrogen.
In view of these problems, there are diaphragm and bellows-type valves, such as that described above, which do not require adjustable valve-spindle seals and are intended to ensure permanent leak-tightness between the process space and the atmosphere. These valves require high displacement forces, because the diaphragms or bellows consist of metallic materials, so that the high process pressures of, for example, pressures above 300 bar, can be withstood. If, in addition to the high pressure differences, high switching frequencies of, for example, 10 to 60 switches per minute, are also required, diaphragms and bellows are exposed to high degrees of alternating stress. The alternating stress generates high material tensions in the material of the diaphragms or bellows, so that, in the case of miniaturized valves, material fatigue and therefore failure of the metal seal components quickly occur. The mechanical stress on the diaphragms and bellows can be reduced by increasing their dimensions and thereby reducing their specific load. The design-based increase in size of the diaphragm or bellows requires an increase in the effort for the valve-spindle movement, so that the construction size of the actuating drive and therefore of the controllable valve increases. Valves of this type are not suitable for compact miniaturized test installations.
A further problem is the leak-tightness of the valve seat. Known valves seal off in the passage over a concentric area. Large-area sealing-off requires high pressure forces in order to achieve gas-tightness in the case of high prevailing pressure differences of, for example, higher than 300 bar. The closure of the flow passage is usually formed from the seat in the valve housing and from the lower part of the valve spindle. The sealing seat in the housing is in this case stationary, and the lower part of the spindle, what is known as the spindle tip, is pressed into the sealing seat of the housing by means of the actuating drive, so that the flow passage is closed. Horizontal large-area sealing-off of the flow passage balances a parallel axial shift between spindle and housing seat within small ranges, so that low manufacturing tolerances are compensated. The spindle tip and the valve seat form a sealing exactly fitting closure point, particularly when the valve-spindle tip possesses a softer material sealing ring and the soft material assumes the form of a counterface of the housing sealing seat as a result of plastic deformation. If high switching frequency leads to increased wear in the region of the spindle seal, the original low position tolerance is increased appreciably and the spindle tip can no longer move into the exactly fitting sealing seat first formed and close the valve passage in a leak-tight manner. This insufficiently fitting closure after the first wear in the sealing region of the spindle can be balanced again by the actuating drive having larger dimensioning. With increased force, a new subsequent deformation of the softer sealing material at the spindle tip becomes possible. Identical actions take place when a softer material is used in the valve seat instead of at the valve-spindle tip. The valve types thus produced have only a short service life in the case of a high differential pressure applied by the gas and in the case of frequent rapid switching cycles.
Many flow passages or valve sealing seats are configured in such a way that sealing takes place over a concentric line. In these instances, too, material combinations are the state of the art. Although such versions of the valve sealing seats require lower closing forces, so that the necessary actuating drive can be small, the positional sensitivity with regard to an angular offset between valve spindle and sealing seat is nevertheless increased substantially. The least possible axial variation in the region of the valve-spindle guide and the spindle seal changes the position of the concentric linear seal and results in leakages in the flow passage.
Investigations on various commercially available valves confirm that, in the case of a hydrogen load under a pressure of up to 300 bar, with high pressure differences between the two sides of the valve, i.e., between the inlet and outlet, there was initially high leak-tightness. On account of the rapid and high switching frequency, the valves showed first leakages after fewer than 10,000 load cycles. Those valves which were able to remain leak-tight even after a larger number of switching cycles had substantially larger drive and large valve forms of construction, and therefore the use of such valves in miniaturized installations would be too complicated and too cost-intensive.
The invention is therefore based on the following object: A pneumatically actuated valve is to be found, which is gas-tight, for example, under extreme process requirements, for example a pressure higher than 300 bar, and with hydrogen as process gas, and, at the same time, a high differential pressure of, for example, higher than 100 bar, even in the case of a number of at least 100,000 switching cycles. The valve is to have a form of construction that can be miniaturized. In addition, the switching time required for an OPEN/SHUT movement of the valve should be extremely short. The valve is to have low wear under continuous maximum load. In special applications, there is, in particular, to be the possibility of using a spring in the pneumatic head, without changing the degree of miniaturization, so that, in the event of the failure of control air (driving air), the valve assumes a predetermined safety, i.e., “fail safe”, position. The set-up of the valve is to compensate for manufacturing and position tolerances as far as possible, so that, along with a high degree of miniaturization, valve costs can be greatly reduced.
Preferably, the design is to be repair-friendly and maintenance-friendly, so that, in the event of valve failure, the valve can be easily and simply repaired. The valve is to be capable of being produced in a simple way, so that, even in the manufacture of customized individual valves, there is a marked cost benefit, as compared with commercially available valves.
In chemical processes, corrosion-resistant materials in the form of high-grade metal alloys are often used, and therefore there is also the requirement for a cost-effective valves consisting of similar corrosion-resistant materials, such as, for example, of Hastelloy™ nickel-based alloys.